The disclosure relates to a power-split transmission for a traction drive and to a method for controlling the transmission.
Power-split transmissions of the generic type for a traction drive, in particular of a mobile working machine, for example of a wheel loader, have not only a mechanical power branch which can be operated with a fixed transmission ratio or with a variable transmission ratio but also a further power branch. The latter is embodied as a hydraulic transmission branch having a hydraulic pump and a hydraulic motor in a closed hydraulic circuit. At least the hydraulic pump is embodied here with an adjustable displacement volume, with the result that the entire transmission ratio of the power-split transmission is infinitely variable by means of the hydraulic transmission branch. Both power branches can be combined here by means of a summing section of the transmission and in this way they act on a common output shaft of the transmission. Both power branches can be coupled or are coupled via a transmission input shaft to a drive machine of the traction drive, for example to a combustion engine, in particular a diesel engine.
In previous transmissions, the displacement volume has been adjusted by means of an electromagnetically activated 4/3-way proportional directional control valve with mechanical feedback, which valve is actuated electromagnetically by a control device of the transmission or of the traction drive. For example, an accelerator pedal or a joystick of the traction drive is connected as a setpoint value transmitter of a velocity to the control device. The setpoint value of the velocity determines here the actuating current which is transferred to the 4/3-way proportional directional control valve, and in this way the quantity of control pressure medium which is applied to an actuating device in order to adjust the displacement volume of the hydraulic pump. Feedback of the current velocity to the control device takes place by means of a rotational speed sensor. Power-split transmissions of the generic type are presented, for example, in laid-open patent applications DE 10 2007 037 107 A1 and DE 10 2007 037 664 A1.
A purely hydraulic drive range in which the transmission of power takes place only via the hydraulic power branch and the mechanical power branch is decoupled is particularly suitable for a working operating mode of the vehicle in which only low speeds are required but a drive torque has to be adjustable over wide ranges. A power-split drive range or a purely mechanical drive range are, on the other hand, predominant suitable for the drive operating mode with restricted dynamics with respect to velocity and traction force or with a virtually constant operating point.
A disadvantage of the known solutions is that the transmissions cannot be adapted flexibly enough to these different requirements of the drive ranges. It is therefore possible, for example, for suddenly increasing loads to occur in a drive range with a low speed if a wheel loader penetrates heavy bulk material with its shovel. Actuating times of conventional, electro-proportional volume flow controllers are then frequently too slow to prevent the drive machine from stalling at this relatively unstable operating point.
An alternative, likewise customary way of controlling the control pressure of the pump makes it possible to react to such load increases but gives rise to a limited controlling capacity of the speed in the power-split ranges.
In contrast with the above, the disclosure is based on the object of providing a power-split transmission for a traction drive, which can be adapted better to drive-range-specific requirements. Furthermore, the disclosure is based on the object of providing a control method for such a transmission.